An RCD socket is a standard power outlet that incorporates an integral residual current device (RCD) that adds ground fault protection to its standard features. An RCD unit is an electrical device which prevents electric shocks, fires, and equipment damage by automatically cutting the supply of power to a circuit if an imbalance between the live and neutral conductors is detected. Many industrial and domestic electrical installations include a central RCD unit that protects all outlets in the installation. The RCD socket is an extremely sensible addition to those installations having no ground fault protection beyond fuses or individual circuit breakers. There are two basic types of RCD-enabled sockets: the fixed type, which is permanently mounted in a wall, and portable units, which plug into non-protected outlets.
Electrical short circuits between live-and-neutral and live-and-ground are among the most common causes of fires, equipment damage, and often fatal shocks. A short circuit that does not initiate an immediate disconnection of the supply can cause sparking, extreme local temperature increases, and potential electrocution. Correctly-rated circuit breakers and fuses generally do a good job of preventing these dangerous conditions, but often are not sensitive, or quick, enough. Negligence or lack of knowledge can also lead to fuses being bridged with links that are too heavy, often resulting in catastrophic electrical accidents. The installation of a central RCD or of RCD socket units can eliminate this danger by offering instantaneous short circuit protection.
The heart of an RCD socket is a hollow, circular, ferrite core through which the socket's live and neutral conductors pass. Wound around the walls of the core is a coil of wire that is attached to an electromagnetic sensor and an interlock mechanism mechanically connected to the socket's main switch. The ferrite core and wire coil serve as a current transformer that maintains a secondary voltage below the sensor threshold if the current between live and neutral remains balanced. In the event of a short circuit, this current will rise rapidly and increase the secondary voltage to a point where it exceeds the sensor threshold, activating the interlock solenoid, and switching the power off. All of this happens in an incredibly short time period, effectively preventing sparking, overheating, and any chance of a shock that could drive the victim's heart into ventricular fibrillation.
The two most commonly encountered forms of RCD socket are the fixed and potable types. Fixed types are similar to conventional wall-mounted outlets with the RCD unit built into their casings. The portable type also has an internal RCD unit, but resembles an adapter with pins on its lower side that plugs into an unprotected outlet and an identical socket on the face of the unit. This allows the user to add RCD protection to any outlet at will. Both types typically feature a test button that introduces a controlled short to the circuit allowing the unit to be tested before use.